Investigations Of Microstructure And Mechanical Properties Of Boron Nitride Fibers

Due to its excellent dielectric properties,stable high-temperature mechanical properties,and a decomposition temperature of 3000°C,boron nitride(BN)fiber has become one of the best reinforced fibers for high-temperature-resistant wave-transmitting ceramic matrix composites.However,the theoretical and experimental research on the mechanical properties of BN fibers at home and abroad is still in its infancy,and there is a lack of effective means to improve the mechanical properties of fibers.Therefore,this paper uses experimental characterization to obtain the microstructure of BN fiber,and on this basis,establishes molecular dynamics and finite element numerical models of BN fiber under different scales to systematically study the microstructure and mechanical properties of BN fibers,so as to provide guidance and suggestions for the preparation of high-performance BN fibers.The main contents of this paper are summarized as follows:1)Experimental research shows that BN fiber is composed of amorphous boron nitride(a-BN),turbostratic boron nitride(t-BN)and hexagonal boron nitride(h-BN).h-BN crystals with random distribution and orientation are embedded in a-BN or t-BN to form the microstructure of BN fibers,and two typical microstructures,namely,parallel configuration and vertical configuration,are observed.The(002)crystal planes of the h-BN grains is parallel and perpendicular to the BN fiber axis,respectively.2)The microstructure and mechanical properties of h-BN and a-BN are studied by molecular dynamics simulation.For the mono-layer h-BN,its in-plane Young's modulus and fracture strength is about 920 GPa and 125 GPa,respectively,and other factors besides temperature,such as the loading rate of nanoindenter etc.,have little effect on its mechanical properties.The examination of fracture process shows that the thickness of h-BN determines its failure mode,and the failure process of each layer is based on the point defect under the nanoindenter as the starting point,and then multiple point defects merge to form hole defects,and finally the expansion of hole defects leads to the complete failure of h-BN.For a-BN,this paper uses molecular dynamics numerical simulation to establish a series of a-BN with different densities and microstructures,which is dominated by B3N3 hexagonal ring.The microstructure of the a-BN atomic model be analyzed by numerical characterization methods.The results show that the low-density a-BN has interconnected micro-void,while the high-density a-BN has discretely distributed micro-void.Compared with h-BN,which has excellent in-plane mechanical properties,the mechanical properties of a-BN are poor,whereas the Young's modulus and fracture strength is approximately 100 GPa and 40 GPa,respectively.3)Based on the exiperimental observation of microstructure for BN fiber,this paper uses molecular dynamics simulation method to establish a two-phase molecular dynamics model of BN fiber.The corresponding microstructures are the parallel configuration and vertical configuration observed in the experiment,and the tensile simulation is carried out along the fiber axis to study the influence of microstructure on its mechanical properties.This numerical simulation study establishes a direct relationship between the microstructure and mechanical properties of the fiber,which is different from the relationship established by experimental characterization and observation.The research results show that the influence of the orientation of grains,crystallinity and grain size on the mechanical properties of BN fiber decreases successively,wherein its grain orientation play the key role determining the level of mechanical properties of BN fiber.According to the orientation of grain,the crystallinity can either increase or decrease the mechanical properties of BN fiber.Among them,the effect of grain size is the smallest.4)Regardless of other forms of BN phases in BN fibers,three types of models of BN fibers composed of h-BN grains are established by using the representative volume element(RVE)method,which are the grain growth(GG)model,the banded grain(BG)model and the uniform grain(UG)model.Various RVE models have sub-micron grains with different geometric shapes.The research results show that,similar to the results of the nanoscale BN fiber atomic model,the grain orientation plays a decisive role in the mechanical properties of the BN fiber,and the improvement of the mechanical properties brought about by the increase in the degree of orientation is much higher than the grain shape and grain size.In addition,the stress analysis found that an obvious stress concentration occurs on the grain boundaries of larger grains,which indicates that the distribution range of grain size has a more obvious influence on the stress distribution.Based on the above research,the most critical issue for the preparation of high-performance BN fibers is how to make h-BN grains oriented along the fiber axis,and whether the increase in crystallinity can improve the mechanical properties of the BN fibers is determined by the orientation's degree of the grain.In addition,controlling the shape and size of h-BN crystal grains to avoid large-size crystal grains can improve the strength of the fiber.